Dual bimetal switches for electrical heaters



N. LAING 3,229,064

DUAL BIME'IAL SWITCHES FOR ELECTRICAL HEATERS Jan. 11, 1966 4Sheets-Sheet 1 Filed Jan. 8, 1962 Jan. 11, 1966 N. LAlNG 3,229,064

DUAL BIMETAL SWITCHES FOR ELECTRICAL HEATERS Filed Jan. 8, 1962 4Sheets-Sheet 2 if j? p/ Jan. 11, 1966 N. LAING 3,229,064

EATERS Filed Jan. 8, 1962 4 Sheets-Sheet 5 N. LAING 3,

DUAL BIMETAL SWITCHES FOR ELECTRICAL HEATERS Jan. 11, 1966 4Sheets-Sheet 4.

Filed Jan. 8, 1962 LVEUIOQ W mam/5 vs United States Patent J s 3 Claims.(Cl. 200-122) This invention relates to thermal switches.

Cheap thermal switches sometimes fail to operate: in some applicationsfailure to operate could have disastrous consequences. Now to improve.reliability a precision-built thermal switch can be used, but inapplications where cheapness is a paramount consideration this again hasits disadvantages.

The object of the invention is to provide a thermal switch which whileit can be made with little more expense than cheap switches commonly inuse, gives a greatly improved reliability.

The invention accordingly provides a thermal switch comprising at leasttwo bimetallic strip portions, a set of contacts associated with eachsuch portion which sets of contacts are connected or adapted to beconnected in series.

The bimetallic strip portions may be provided by one single bimetallicstrip encastered at a middle section thereof. Alternatively thebimetallic strip portions may be separate and spaced apart so as to besensitive to adjacent environments. In this case each bimetallic stripportion may be set to operate when the temperature reaches a given valuein its environment; however if one fails to operate at the given valueand the temperature of its environment continues to change, the otherwill be affected before any danger arises.

It will be seen that if (say) the chance of one bimetallic strip portionfailing to operate is 10 the chance of both failing is 10'". Evengreater security can be obtained with additional bimetallic stripportions and series-connected sets of contacts, but for most purposestwo strip portions and contact sets will suffice.

Various embodiments of the invention will now be described by way ofexample with reference to the accon1- panying drawings, in which:

FIGS. 1, 2 and 3 are respectively a sectional plan and two othersectional views of a thermal cut-out, the section planes of FIGS. 2 and3 being shown by the lines 2a2a and Zia-3a respectively in FIG. 1;

FIG. 4 is a perspective view of a heater having a further form ofthermal cut-out associated therewith;

FIG. 5 is a side elevation of part of the FIG. 4 heater and cut-out;

FIG. 6 is a partial perspective view of a further heater and cut-out,and

FIGS. 7, 8 and 9 are views of yet another heater and cut-out, FIG. 7being a longitudinal vertical section, FIG. 8 a partial side elevationof one end portion of the heater and cut-out with parts removed, andFIG. 9 an exploded partial perspective of that end portion, shown on agreater scale than FIGS. 7 and 8.

Referring to FIGS. 1 to 3 of the drawing, the thermal cut-out thereshown comprises an insulating rectangular base 1 with an integral flange2 along one long side. A single bimetallic strip 3 is clamped against aspacer 4 located at the mid-point of the base 1 by means of two rivets 5which pass through the strip 3 and spacer. The bimetallic strip 3 isthus divided into two equal strip portions 3a, 1% which are supported ascantilevers and are capable of acting independently. Adjusting screws6a, 6b extend through threaded inserts 7a, 7b set in the base 13,229,064 Patented Jan. 11, 1966 equidistantly on either side and pressagainst the strip portions 3a, 3b respectively. The ends of thebimetallic strip 3 are turned up away from the base 1 to form flanges8a, 8b: the height of the flanges increases going towards the flange 2on the base 1.

A single leaf spring 10 having the form of a bent rectangular strip isclamped over its mid-portion to a thickened middle portion 11 of theflange 2 on the base 1 by two rivets 12: the leaf spring is thus dividedinto two equal and similar portions 10a, 10b which are supported ascantilevers and are capable of acting independently. The ends 13a, 13bof the leaf spring 10 extend beyond the base 1: adjacent the ends 13a,13b strip spring portions 10a, 10b carry moving contacts 14a, 14bcooperating with fixed contacts 15a, 15!) mounted on the flange 2 of thebase 1 near the ends thereof.

In use of the cut-out, a circuit is completed through a current source16, a utilization device 17 such as a heater, the contacts 15a, 15b andthe leaf spring 10, the contact sets 14a, 15a and 14b, 1519 beingseries-connected.

In the normal functioning of the cut-out, the portions of the leafspring 10 adjacent its ends 13a, 13b contact the abutments formed by theadjacent ends of the flanges 8a, 8b: the spring portions 10a, lllb arethus kept stressed and the contacts of each contact set 14a, 15a and14b, 15b are maintained in contact-making relation. When the environmentheats up beyond a predetermined temperature, the bimetallic stripportions 3a, 3b will flex towards the base 1 whereby the ends of theflanges 8a, 8b will clear the leaf spring portions 10a, 1012, which, areno longer constrained, revert to an unstressed position wherein thecontacts of the contact sets 14a, 15a, 14b, 1512 will separate to breakthe circuit. It will be seen that so long as contact is broken at one orother of the contact sets 14a, 15a; 14b, 1512 the utilization device 17will be on open circuit.

Unce contact is broken at either contact set 14a, 15a; 14b, 15b it willremain broken, even if the temperature drops again, until the ends 13a,13b of the leaf spring are pushed towards the fixed contacts 15a, 15b,e.g. by manually controlled push rods (not shown): the end portions ofthe leaf spring then ride over the sloping edge of the flanges 8a, 8b onthe bimetallic strip 3, thereby camming the ends of the stripresiliently towards the base 1, until the initial position is once againreached when the flanges 8a, Sb snap upwardly once more to hold the leafspring portions 10a, ltlb with the movable contacts 14a, 14b against thefixed contacts 15a, 15b.

FIG. 4 shows a heater in the form of an insulating base 20 and ahorizontal rectangular support strip 21 carrying insulating strips 22along either long edge, these insulating strips being notched at regularintervals as shown at 23 to receive the convolutions of a helicallycoiled resistor strip 24; the resistor strip 24 is more or less rigidand has its ends 25, 26 fixed to a base 20. Thus the resistor striphelix mounts the support and insulating strips 21, 22 by reason of theengagement of the latter with the convolutions of the helix, the strips21, 22 helping to rigidity the helix for the same reason.

A bimetallic strip 27 overlies the support strip 21 and is securedthereto by a pair of rivets 28 passing through a central portion 29 ofboth strips: at either side of the central portion 29 the bimetallicstrip 27 is cranked away from the support strip as indicated at 30. Thebimetallic strip 27 is thus divided symmetrically into two portions 27a,27b supported cantilever-fashion and capable of functioning separately.

The ends 31a, 31b of the bimetallic strip 27 are bent downwardly andinwardly and in the normal position of the arrangement (shown in FIG. 4and in full lines in FIG. 5) engage the free ends of leaf springs 32a,

3% the other ends of which are clamped against the ends of the base byscrews 33. The leaf springs 32a, 32b carry moving contacts 34cooperating with fixed contacts 35a, 35b secured on conductive brackets36a, 36b fixed to the base 20, the same screws 37a, 37b as secure theends 25, 26 of the helix serving also to secure the brackets. Theengagement of the bentdown ends 31a, 31b of the bimetallic strip 27stress the leaf springs 32a, 32b and hold the contacts of the contactsets 34, 35 in contacting relationship. When the temperature rises thebimetallic strip portions 27a, 27b flex away from the support strip 21until one or other of the ends 31a, 31b of the bimetallic strip clearsthe end of the corresponding leaf spring 32a, 32b and thereby releasesit: the leaf spring then springs away from the bimetallic strip andbreaks contact between the contacts of the corresponding contact set.This operated position of the bimetallic strip portion 27a and leafspring 32a is illustrated in dotted lines in FIG. 5. The leaf springs32a, 32b and the ends 31a, 31b of the bimetallic strip 27 are shaped sothat by pressing the spring inwardly from its released position its freeend cooperates with the end of the bimetallic strip to push up thelatter by carnining action until the end of the strip is able to dropdown behind the spring to retain it in the normal position previouslydescribed.

An electric power source 38 is connected across the leaf springs 32a,32b so that in the normal position of the apparatus current istransmitted to the resistor strip 24 through the leaf springs 32a, 3212,the contact set 34, 35 and the bracket 36a, 36b; separation of thecontacts of either contact set suflices to open-circuit the helix.

It is assumed that the bimetallic strip 27 is of high resistance;alternatively its ends 31a, 31b can be insulated or it can be formed intwo parts.

FIG. 6 shows a further form of heater comprising a resistor strip Woundinto a helix: the strip is here designated 40 and supported on a pair ofmore or less parallel support strips 41, 42 having insulating edgeportions notched at 43 to receive the convolutions of the helix. Unlikethe previous embodiment the support strips 41, 42 are mounted betweenplate-like end members 44 which are secured to a base (not shown).

The upper support strip 41 mounts at each end a bimetallic strip 45extending generally parallel thereto: one end portion of each bimetallicstrip 45 is riveted to the support strip 41 at 4-6 and the strip 45 iscranked as shown at 47 so that the remainder thereof lies spaced fromthe support strip. The free end of the bimetallic strip 45 extendsthrough a window 48 in the adjacent end member 44 and is turned down toform a sloping flange 49.

A U-shaped spring 50 has the extremity of one arm secured to each endmember 44- at 51, while the extremity 52 of the other arm 53 isoutwardly bent to cooperate with the abutment provided by the larger endof the flange 49 on the corresponding bimetallic strip 4-5. The spring50 is mounted to flex in a plane parallel to the end member 44, whilethe strip 45 is arranged to move in a plane perpendicular thereto: inthe normal positions shown the flange 49 keeps the spring 50 stressedwhereby to maintain contact between a movable contact 55 on the springarm 53 and a fixed contact 56 mounted on the end member 44.

A leaf spring 57 having one end encastered in the end member 44 extendsinto the window 48 and with its free end 58 contacts the top of thebimetallic strip 45 to urge it downwardly. An adjusting screw 59 allowsthe spring pressure on the strip 45 to be adjusted.

The springs 50 at the two ends of FIG. 6 apparatus are connected tocorresponding ends of the helix 40, and the fixed contacts 56 areconnected across an electrical power source (not shown). When thetemperature rises above a pre-set value the strips 45 flex upward sothat the ends of the flanges 49 clear the extremities 52 of the springs50 and allow the latter to spring open, thus break- 4 ing contactbetween the contacts of the two sets thereof 55, 56.

When the cut-out has operated it can be returned to its normal conditionby manually or otherwise moving the spring arm 55 towards the other arm.The extremity 52 rides up the sloping flange 49 on the bimetallic strip45, thereby displacing it upwards, until the extremity has passed beyondthe flange whereupon the latter snaps down to hold it in the positionillustrated ready to operate again.

FIGS. 7, 8 and 9 show an assembly including a heater similar to that inFIG. 6; similar parts are given the numerals used in that figure andwill not need further description.

The end members 44 in the present embodiment are substantiallyrectangular, and are recessed from the top at 69 to provide a seat 61for the corresponding extremity 62 of the upper support strip 41 whichin this instance is of dished section. An insulating spacer 63 isreceived in the recess 66 on top of the extremity 62 and is shaped tomate therewith. A spigot 64 upstanding centrally from the bottom of therecess 66 extends through a mating hole 65 in the extremity 62 and intoa recess (not shown) in the bottom of the spacer 63. The spacer 63carries an integral upstanding spigot 66 aligned with the spigot 64which in turn extends through a hole 6'7 in a bimetallic strip 6% andinto an insulating retainer element 69. Aligned central vertical boresin the element 6 spigot 66 and spacer 63, spigot 64 and end member 44terminating in a slot 79 receive a clamping bolt 71 which is tightenedinto a nut 72 located in the slot 70, whereby to hold the assemblyrigidly in position.

The bimetallic strip 68 has an integral terminal lug 74 extending to theside of the end member 44 away from the helix 40. The part of the strip68 on the side of the helix 40 is formed with a U-shaped cut-out 75whereby to define an internal tongue 76. The free end of the tonguecarries a moving contact 77 which cooperates with a fixed contact '78 onthe support strip 41 connected to one end of the helix. In the normal(FIG. 7) position of the bimetallic strip the tongue 76 presses thecontact 77 firmly against the contact 78 by reason of the springy natureof the strip.

The terminals '74 at opposite ends of the apparatus are connected to anelectric power source (not shown). The helix 4% is thus connectedthrough the bimetallic strips 63 and the two contact sets 77, 78. Whenthe temperature rises above a preset value the tongue 76 of eachbimetallic strip snaps up to the position shown in FIG. 8, therebyopen-circuiting the helix 40. When the temperature drops again thecontacts 77, '78 close automatically.

The heater and cut-out assemblies of FIGS. 4 to 9 are particularlysuitable for use with cross-low blowers (see for example British PatentSpecification 876,611) in which case the blower rotor and helix areequal in length and arranged opposite one another. In such a combinationheat dissipation from the helix is dependent on the flow therethrough ofair from the blower: it is usually considered best for the heater to bedisconnected as soon as any part approaches the temperature when itwould glow red but the apparatus will only be dangerous at a somewhathigher temperature. Each bimetallic strip portion will normally operatewhen the temperature at the adjacent end of the helix reaches the presetvalue. In the worst condition possible flow adjacent one end will beblocked while flow at the other end is normal, and the bimetallic stripportion at the first end will fail. Only slight further heating at theend will produce a rise in temperature at the other end that willoperate the bimetallic strip portion thereof.

1 claim:

1. Apparatus comprising an electric heating element extending over anarea and thermal protective switch means therefor to interrupt flow ofcurrent through the element on overheating thereof; the thermalprotective means comprising two pairs of Contacts series connected witheach other through the heater element, and a first bimetallic strip anda second bimetallic strip located adjacent said heater element andspaced from one another over said area, each bimetallic stripcontrolling one pair of contacts and operative to open a pair ofcontacts when the temperature of the respective strip rises above apreset value.

2. Apparatus as claimed in claim 1, wherein the heating elementsurrounds the major part of each bimetallic strip.

3. Apparatus as claimed in claim 1, wherein the heating element includesa resistance element and support means therefor comprising first andsecond brackets, said pairs of contacts being located one at the firstbracket and the other at the second bracket, one bimetallic strip beingmounted adjacent the first bracket. with a free end terminating at thispoint and controlling the pair of contacts at that bracket, and theother bimetallic strip being mounted adjacent the second bracket with afree 6 end terminating at this point and controlling the pair ofcontacts at the second bracket.

References Cited by the Examiner UNlTED STATES PATENTS 831,847 9/1906Erickson 200-122 1,323,477 1/1920 Baker 200122 1,435,580 3/1924Zimmerman 200-116 1,550,641 8/1925 Whittingham 200122 1,678,401 7/1928Luttrell 200-122 1, 48,938 2/1934 Lawton 200-122 2,016,244 10/1935Gregory 200-l22 2,414,531 1/ 1947 Johns 200122 2,478,541 8/1949McCracken 200-122 2,528,591 11/ 1950 Getchell 200-122 FOREIGN PATENTS418,810 10/1934 Great Britain.

BERNARD A. GLLHEANY, Primary Examiner.

1. APPARATUS COMPRISING AN ELECTRIC HEATING ELEMENT EXTENDING OVER ANAREA AND THERMAL PROTECTIVE SWITCH MEANS THEREFOR TO INTERRUPT FLOW OFCURRENT THROUGH THE ELEMENT ON OVERHEATING THEREOF; THE THERMALPROTECTIVE MEANS COMPRISING TWO PAIRS OF CONTACTS SERIES CONNECTED WITHEACH OTHER THROUGH THE HEATER ELEMENT, AND A FIRST BIMETALLIC STRIP ANDA SECOND BIMETALLIC SRTIP LOCATED ADJACENT SAID HEATER ELEMENT ANDSPACED FROM ONE ANOTHER OVER SAID AREA, EACH BIMETALLIC STRIPCONTROLLING ONE PAIR OF CONTACTS AND OPERATIVE TO OPEN A PAIR OFCONTACTS WHEN THE TEMPERATURE OF THE RESPECTIVE STRIP RISES ABOVE APRESET VALUE.